As organizations continue to downsize, merge with other organizations, and otherwise restructure, the following questions become increasingly significant: What impact do such organizational changes have on the job security of individual workers? What are the cognitive, affective, and behavioral consequences of reduced job security? What other factors might serve to alter perceptions of job security? An integrated model based on person-environment fit theories of stress (e.g., Everly & Sobelman, 1987; Lazarus, 1966), theories of job adaptation (e.g., Hulin, 1991), and Weiss and Cropanzano's (1996) Affective Events Theory is proposed which conceptualizes job insecurity as an organizational stressor. The relationships between the multiple hypothesized antecedents and consequences of job security are predicted to be mediated by an individual's job attitudes and job affect and moderated by a number of individual differences variables---specifically, job importance, self-efficacy for adapting to organizational transition, negative affectivity, and perceptions of procedural justice. Measures of perceptions of job security and satisfaction with job security are developed by means of classical test theory as well as item response theory. Finally, two tests of the job security model are undertaken. The first, a field study, examines the individual and organizational consequences of an organizational restructuring involving the merger of five government agencies. The second, a laboratory experiment, focuses on individuals' psychological and behavioral responses to downsizing in a simulated organization as a function of the individuals' level of job security and procedural justice of method for determining layoffs. Results indicate that perceptions of job security are derived from multiple individual and organizational antecedents, including organizational change, job technology change, and several worker characteristics. Results also indicate that job insecurity has a significant impact on individual and organizationally relevant outcomes, specifically, job attitudes, job affect, psychological and physical health, organizational commitment, and organizational withdrawal tendencies. In addition, two hypothesized variables were found to consistently moderate the impact job insecurity has on these cognitive, affective, physical, and behavioral outcomes---namely, (a) self-efficacy for adapting to organizational transitions and (b) ob importance. Theoretical and practical implications of the results are discussed. Finally, directions for future research are proposed.